The present invention relates to a broach assembly and method of using the broach assembly for refurbishing generator field wedge form slots.
As the rotor of an electrical generator rotates, the generator field coil slots and wedge forms can deteriorate and distort during operation. When maintenance is performed on the coils of a generator in the field, the wedges which contain the coils in the slots during operation are removed. Numerous problems can occur as a result of the disassembly. Galling, burns and slot distortion are common. Also, the field coil slots may distort during operation.
To cure these problems, factory milling has heretofore been necessary depending upon the severity of the problem. More commonly, extensive and time-consuming manual work is necessary to refurbish the slots in order to accommodate new or used wedges. Also, remachining of wedge involves large, highly specialized machine tools, not commonly found in the industry. Further, where field coil slots distort during operation, it is not uncommon to require machining of the slots to an oversized condition, necessitating replacement of the wedges, inherently resulting in higher operational stresses. Commonly, hand tools and numerous man-hours were required to assure proper wedge contact and assembly sliding fit prior to reassembly.
In accordance with a preferred embodiment of the present invention, there is provided a broach assembly which can be readily and easily passed through the slots to refurbish the active contact surfaces of the slots. It will be appreciated that the active contact surfaces are those surfaces which generally face radially inwardly of the slot and lie in contact with the generally radially outwardly directed active contact surfaces of the wedges as a result of the rotation of the core relative to the frame. The inactive surfaces of the slots face generally radially outwardly, while the inactive surfaces of the wedges face radially inwardly. These inactive surfaces do not generally engage one another during operation. It will be appreciated that the slots of the rotor core are also generally dovetail-shaped slots with inclined active surfaces and generally inactive tangential surfaces on opposite sides of the slot.
The broach assembly hereof is formed of multiple parts, including an elongated base and a plurality of tool holders mounted in longitudinally spaced relation to one another along the base. Preferably, bolts are employed to bolt the tool holders to the base. Each tool holder carries a pair of cutters along respective opposite sides of the holder, the cutters having cutting surfaces for engaging the active surfaces of the slot when the broach assembly is passed through the slot. The base includes an elongated body having a pair of flanges extending laterally to opposite sides of the base. The undersurfaces of the flanges of the base are preferably tapered from a leading end to a trailing end of the assembly such that the elevation of the broach assembly increases as it is passed through the slot. That is, the cutting surfaces of the cutters are progressively spaced in a radial outward direction caused by the cooperation of the tapered surfaces of the base and the inactive surfaces of the slot, enabling the active surfaces of the slot to be progressively cut or refurbished.
In a preferred embodiment of the present invention, shims are disposed between the flanges of the base and the flanges of the tool holders. By using increasingly thicker shims and multiple passes of the broach assembly through the slot, the cutting surfaces are progressively displaced radially outwardly to further refurbish the active surfaces of the slot. Alternatively, multiple shims can be utilized between the flanges of the base and flanges of the tool holders on each of the opposite sides of the broach assembly.
By passing the broach assembly through the slots, preferably using multiple passes with progressively increasing displacement of the cutting surfaces in a radial outward direction, the inconsistencies and interferences along the active surfaces of the slots are removed or minimized. Preferably, the broach assembly rides on the inactive portions of the wedge form slots and is lubricated to limit friction and galling. The broach assembly can be passed through the slots by many different devices, for example, hydraulic, pneumatic or electric motor drives, chain drives and the like. By passing the broach through the slots at the designed clearance, gross obstructions and distortions are removed. Additional material can be removed by the addition of the precision shims to the broach assembly.
In a preferred embodiment according to the present invention, there is provided a broach assembly for establishing active contact surfaces in a dovetail slot of a generator core, comprising a pair of tool holders, a pair of cutters having cutting surfaces carried by and disposed along opposite sides of each tool holder, an elongated base having surfaces for engaging inactive surfaces of the dovetail slot and having leading and trailing ends thereof, the tool holders being secured to the base at spaced longitudinal positions therealong, the surfaces of the base being tapered in a direction from the leading end of the base toward the trailing end thereof for engaging the inactive surfaces of the slot, enabling the cutting surfaces of the cutters to progressively cut the active contact surfaces of the slot as the assembly passes through the slot leading end first.
In a further preferred embodiment according to the present invention, there is provided a method of progressively cutting active surfaces in a slot of a first part to establish contact with mating surfaces of a second part receivable in the cut slot, comprising the steps of providing an assembly of tool holders on a base with each tool holder having a pair of cutters along opposite sides thereof for cutting material from the active surfaces of the slot, forming tapered surfaces along opposite sides of the base for engaging inactive surfaces of the slot, the base having leading and trailing ends, disposing a pair of shims along opposite sides of the base and passing the assembly through the slot leading end first with the cutting surfaces engaging the active surfaces in the slot and in a direction enabling the cutting surfaces of the cutters to progressively cut the active surfaces of the slot to form active contact surfaces for engaging the mating surfaces of the second part upon reception of the second part in the cut slot of the first part.